Most existing Wireless Local Area Network (WLAN) systems based upon OFDM modulation techniques comply with the IEEE 802.11a/g standard (see, IEEE Std 802.11a-1999, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification: High-speed Physical Layer in the 5 GHz Band”). In order to support evolving applications, such as multiple high-definition television channels, WLAN systems must be able to support ever increasing data rates. Accordingly, next generation WLAN systems should provide increased robustness and capacity.
The receiver of an OFDM system can be based on a number of RF architectures, including the heterodyne, homodyne zero-IF and homodyne low-IF architectures. A heterodyne receiver utilizes a two-step process to regenerate the original baseband signal. The homodyne zero-IF design utilizes a single step process to create the baseband signal. The homodyne low-IF method utilizes a basic two-step process, but the intermediate frequency signal created after step one is at a very low frequency and is treated as a type of baseband signal.
It is generally recognized that the best choice for an efficient, integrated receiver is the homodyne zero-IF architecture. While all of the architectures described above suffer from some degree of DC offset at the receiver, the homodyne zero-IF architecture generates the most DC offset (in addition to other impairments). The OFDM modulation technique, however, is especially sensitive to a DC offset in the received signal. The sensitivity is a function of the data rate, which is itself a function of the constellation and coding over the subcarriers. While there are a number of design techniques that can be utilized to mitigate or reduce the effects of DC offset (e.g., DC offset calibration or AC coupling), it remains difficult to meet the requirements of the highest data rate OFDM modulation specification (64 QAM at 54 Mbps).
A need therefore exists for a method and system to estimate the DC offset in an OFDM receiver. A further need exists for methods and systems to compensate for such DC offset.